The invention relates to a metallic material with additives, advantageously non-metallic additives, embedded therein, more particularly a contact material for electric contacts. Advantageous methods for producing such a material are also disclosed.
Silver and its alloys are predominantly used as material for electric contacts because apart from having a very high electric conductivity these contact materials have an adequate resistance to oxidation and favourable properties regarding transfer resistance. Known contact materials based on silver are subject to an undesirable tendency to weld, for the contact surfaces to stick to each other or for material to migrate between contact members, and an excessive burning rate in the case of highly loaded switching contacts which are subject to arcing in the course of the switching operation.
To overcome these disadvantages the contact materials have been improved by additions to the silver or silver alloy parent material. Known additives include metal oxides such as cadmium oxide, stannic oxide, magnesium oxide, zinc oxide, lead oxide and the like which are embedded in finely divided form in the parent material. Contact materials of this kind have a reduced burn-up, reduced welding in switching and a relatively low contact resistance. Oxides are introduced into the silver parent material mainly in accordance with two known methods:
Powder metallurgy in which silver powder and metal oxide powder is compressed and subsequently sintered; and PA1 Internal oxidation in which an alloy of silver is annealed in an oxidizing atmosphere with a baser metal, for example cadmium, zinc or tin. In this latter method the oxygen diffuses into the interior of the alloy and reacts with the baser alloying constituent while forming an oxide distribution in fine dispersion.
Powder metallurgical production of the contact material may be accompanied by irregular mixing of the pulverized constituents. Furthermore, compression and sintering operations may be subject to demixing which in turn results in concentration fluctuations. Materials produced by powder metallurgy also give rise to specific difficulties in chipless forming because they have only a low ductility, particularly if the non-metallic components represent a high proportion.
If the contact materials are produced by the method of internal oxidation the concentration of the metal oxide will be defined by the solubility of the alloying metal in silver and by the appearance of the so-called external oxidation. This phenomonen prevents further ingress of oxygen due to the formation of oxygen-impermeable cohesive oxide films and thus prevents selective oxidation of the baser alloying constituents in the interior of the silver parent material. For example, it is not possible to subject alloys containing more than 2% by weight of tin to internal oxidation. The kinetics of internal oxidation moreover give rise to a particle size gradient of the oxide extending from the surface into the interior of the material. As a consequence it is not possible to maintain the required uniformity of oxide particle size.